1. Field of the Invention
The present invention relates generally to downhole tools for use in wellbores. More particularly, this invention relates to an isolation assembly for use with coiled tubing operations, such as the pressure testing, matrix stimulation, or fracturing (“fracing”) a well with a downhole tool, among other things.
2. Description of the Related Art
In the drilling and production of oil and gas wells, it is frequently necessary to isolate one subterranean region from another to prevent the passage of fluids between those regions. Once isolated, these regions or zones may be fraced or injected with a formation compatible fluid as required to stimulate production of hydrocarbons from the zones. Many stimulation techniques for given types of wells are better suited to using coiled tubing, as opposed to conventional jointed pipe intervention. Generally, it is known to attach a selective isolation device, such as a straddle packer, to coiled tubing and run the packing device downhole until the desired zone is reached. Once positioned, prior art fracing fluids or stimulation fluids may be forced into the zone.
In many downhole coiled tubing operations, it is known to use a check valve. Prior art check valves include the dual flapper back pressure valve product family H13204 from Baker Oil Tool, for example. Other types of check valves currently used include a ball and dart type check valve known to those of skill in the art. Generally, these check valves operate to provide a safety or control measure to prevent wellbore pressure from entering the coiled tubing work string. This feature is especially important when utilizing coiled tubing, as an unexpected surface failure of the coiled tubing may result in a surface release of wellbore fluids. The check valves are normally run directly below the coiled tubing connector.
However, in most operations, the check valve will not allow pressure below the check valve to be bled off at surface through the coiled tubing. Thus, for some operations performed with coiled tubing, it is not possible to utilize a check valve. For instance, fracturing operations using coiled tubing generally require that check valves are not run. This results in a potential flow path for the fluid in the coiled tubing and the formation (i.e. an uncontrolled release up the coiled tubing string) should the coiled tubing fail on surface. The resulting situation has a high potential of injury to personnel and other damage that may result in compromising well control. Thus, it is desirable to provide a check valve when fracturing or otherwise treating with coiled tubing to confine the potential release to just the coiled tubing volume above the check valve, such that the considerable volume of fluid in the formation would remain isolated.
Other operations (e.g. reverse circulating) cannot be performed efficiently utilizing a check valve with coiled tubing. For instance, and by way of example only, a traditional pressure test for a straddle packer cannot be performed when a check valve is used with coiled tubing. Generally, before beginning a fracturing or stimulation operation, a straddle packer is set in unperforated casing to ensure the integrity of the seal of the cups of the straddle packer. As shown in FIG. 1, straddle packers 10 are known to be comprised of two packing cups 11 and 12 mounted on a mandrel having a port 15. To test the integrity of the straddle packer 10, the straddle packer 10 is run into a non-perforated section of the wellbore casing 10, generally below the perforated zones.
To energize the straddle packer 10, pressurized fluid is pumped from surface through the coiled tubing 1, into the mandrel of the straddle packer, and out flow port 15 between cups 11 and 12. If the cups 11 and 12 function properly, the packer 10 will be set in the casing.
Once the integrity is ensured, the pressure in the coiled tubing is bled off, and the pressure between the cups 11 and 12 forces the fluid back into the coiled tubing to concomitantly reduce the pressure within the cups 11 and 12, as the straddle packer 10 is in direct communication with the coiled tubing. The straddle packer is then de-energized and free to move uphole to the perforated zone 5 to be fraced.
If a check valve were located between the straddle packer 10 and the coiled tubing string 1, then the pressure within the straddle packer 10 would create a pressure differential across the check valve such that the check valve would remain closed. Thus, no direct communication path would exist for the fluid to exit the straddle packer 10 and the straddle packer 10 would become fixed in the casing.
As stated above, fracing operations and other stimulation operations cannot be performed utilizing a check valve while a cup-type selective isolation device is used. Thus, when fracing or otherwise stimulating the well, it may be desirable to completely bleed down the pressure within the coiled tubing prior to attempting to move the tool below for various reasons, such as to improve the fatigue life of the coiled tubing and to improve the safety of the operation. However, in most applications, this 100% bleed down at surface is not commercially feasible, as the formation has been energized by the stimulation operation, and fluid communication is provided from the formation to surface. Thus, a complete bleed down at surface would require excessive time to complete, depending on the state of the formation.
In some applications, after each treatment, the coiled tubing may be bled down to allow the downhole tool to be re-positioned over another zone or pulled to surface. This may allow hazardous formation gas and fluids to enter the tubing, if no check valve is utilized. However, due to time constraints, in some operations the pressure in the coiled tubing is not bled down to be equalized with that in the annulus or completely bled down to atmospheric pressure (zero internal pressure in the coiled tubing). For instance, applied pressure within the coiled tubing may remain between 600 and 1000 p.s.i. while moving the packer. This may increase the wear on the cups 11 and 12 of the straddle packer 10. Further, it has been found that winding the coiled tubing on the spool at surface while the coiled tubing experiences these internal pressures may significantly accelerate the fatigue experienced by the coiled tubing and decreases the operational life of the coiled tubing, as shown on Table 1 described hereinafter. Thus, it would be desirable that the coiled tubing be allowed to be more completely bled down at surface prior to repositioning the downhole tool. In this way, pressure within the coiled tubing will not excessively fatigue the coiled tubing string as the string is wound around the spool at surface.
Thus, it would be desirable to provide an assembly for a downhole tool that would allow a check valve to be utilized in various downhole applications, such as when setting and using a straddle packer in fracing or other operations. Such an assembly would improve the operational life of the coiled tubing string, as well as increase the speed of performance of the given function, as operators at surface would not have to wait until the entire coiled tubing and formation are bled down from the straddle packer to surface. Finally, such a check valve would significantly increase the safety associated with performing these downhole operations.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the issues set forth above.